1. Field of the Invention
The present invention relates to a system and method for creating thrust reversal by means of two sets of counter rotating propellers, and more particularly to such a system and method used where the propellers are driven from an engine, such as a free-turbine engine, where both of the propeller rotors should be run at roughly equal speed and positive power levels.
2. Background Art
One method of creating thrust reversal by means of variable pitch propeller blades is to change the pitch of each blade so that the angle of attack of the blade is such so as to create a reverse thrust. However, where there are counter-rotating propellers, with forward and rear sets of propeller blades being positioned closely adjacent to one another, the creating of reverse thrust by setting the propeller blades of the forward set at a negative angle has the effect of "blanking out" the air flow to the rear propeller blades. This compromises the ability of the rear set of propeller blades to absorb power.
This situation causes a problem in reversing the thrust of counter-rotating propellers where the design of the engine (and gearbox, if any) does not mechanically constrain the two rotors to turn at the same rpm (or the same ratio of rpms). Such designs are the most common and desirable, and include schemes in which each rotor is driven by the inner or outer portion of a counter-rotating statorless turbine (wherein the only connection between rotors is via the gas path), or in which a differential planetary gear box is used. (The kinematics of the latter will permit one rotor to slow down in a fixed relation to the speeding up of the other.) In either case, if the aerodynamic torques exerted by the two rotors on the passing air stream are not held to a fixed ratio to each other, the more lightly loaded rotor will accelerate. The more heavily loaded rotor (i.e., the one producing the greater thrust) will suffer performance degradation because overall engine power must be reduced to prevent overspeeding the lightly loaded rotor.
A search of the patent literature has disclosed a number of patents which deal with variable pitch propellers, devices which control the relative rates of rotation of propeller sets, and also thrust reversal by means of fan or propeller blades. It is believed that none of these are closely relevant to the problems addressed by the present invention and are not particularly relevant to the particular invention itself. However, these are being cited to ensure that there is a full disclosure of all possibly relevant prior art known to the inventor. These are as follows:
U.S. Pat. No. 2,126,221 (Sessums) shows a propeller unit for an aircraft where there are forward and rear propellers mounted to the same driveshaft. The forward propeller is set at a greater pitch and the rear propeller at a lesser pitch. During takeoff and climb, the forward propeller is disengaged from the drive shaft so that it rotates freely, while the rear propeller provides the thrust. At cruise, when the plane is traveling at a greater speed, the front propeller is cause to engage the drive shaft so that it now creates thrust. The purpose of this arrangement is to avoid the need of variable pitch propellers and yet enable the propeller unit to operate efficiently at the takeoff and climb mode, and also the cruise mode.
U.S. Pat. No. 2,216,013 (Kenney) shows a torque balancing mechanism to equalize the loads on the two shafts that drive counter rotating propellers. If the load on one propeller is greater than the load on the other, then this is sensed by a gear mechanism which is attached to the two counter-rotating shafts and which moves a control shaft or rotor 5 to equalize the load on the propellers. In the particular embodiment shown herein the load is equalized by changing the pitch of one of the propellers.
U.S. Pat. No. 2,228,638 (Mercier) shows a gear drive system for counter-rotating propellers where the load shared by the propellers is equalized due to the action of the gear system. In one arrangement, there are two planetary gear systems coaxially mounted and spaced axially from one another with the drive shaft driving the planet gears of a first planetary system, with these planetary gears in turn driving a ring gear to rotate the first propeller, and also rotating the sun gear of the second planetary system which in turn delivers power to the planet gears of a second planetary system. The planet gears of the second planetary system are fixed to stationary structure and the ring gear of the second planetary system is connected to the second propeller.
U.S. Pat. No. 2,455,239 (Doussein) shows a device for regulating the pitch of two counter rotating coaxial propellers in a manner that the torque on each propeller is equal to that on the other. There are two drive shafts which are connected to their respective propellers through elastic devices which are in turn connected to an electrical differential relay. The differential relay in turn controls an electromagnetic clutch arrangement provided between the pitch control device and one of the propellers. Thus, the load on the two propellers remains equal.
U.S. Pat. No. 2,533,346 (Brady, et al) shows a dual rotating propeller system where the propellers each have their pitch varied by means of a single power source. A particular gear trained to accomplish the variable pitch function is described.
U.S. Pat. No. 2,584,115 (Dinsmoor) shows a torque equalizing mechanism for counter rotating propellers where the pitch of the blades of one propeller is automatically adjusted in a direction to equalize the torque on the propellers in response to a difference in the speeds of the propellers. This is done by means of a speed-changing shaft positioned within the propeller shafts to change the propeller pitch.
U.S. Pat. No. 2,607,430 (Dean) relates to fans used in conjuction with aircraft propellers. The fan blades are variable in pitch so that the effect of the fan may be changed in accordance with airplane speed or fan speed conditions, and a mechanism is provided which permits the fan blades to be changed in pitch coincidentally with the changes in pitch of the propeller blades.
U.S. Pat. No. 2,674,407 (Ostrognai) describes what is called "a free air fan." There is a primary fan having a set of blades of a fixed pitch and a secondary fan having a set of blades at a different pitch mounted on the same axis as the primary fan. The blades of the primary fan are set at a pitch to draw air toward the blades of the secondary fan, and the blades of the secondary fan throw the air in the desired direction, depending upon the angle of set or pitch. By different relative angles or pitch of the blades of the primary and secondary fans, a choice in the cone or pattern of air flow of the fan may be obtained, varying from a narrow axial stream to a wide radial flow.
U.S. Pat. No. 2,982,361 (Rosen) discloses a propeller where there are two banks of blades with each blade of one bank cooperating with the blade of the next bank so as to form together cooperating elements of one effective air foil for takeoff and two separate air foil elements for cruise.
U.S. Pat. No. 3,830,587 (Shipes, et al) shows an axial flow fan assembly for use in air coolers and other industrial environments. This is a series fan assembly in which the fan stages are mounted closely together, and the blades of the downstream fan have a greater average pitch angle than the blades of the upstream fan.
U.S. Pat. No. 3,986,794 (Korn) shows a reversible ducted fan assembly where each of the blades or air foils of the fan have first and second portions which are rotatably supported from the fan hub and are moveable from a forward thrust position when the trailing edge of the first portion is positioned adjacent to the leading edge of the second portion to a reverse thrust position wherein the first and second portions are both rotated so as to locate the leading edge of the first portion adjacent to the trailing edge of the second portion. The effect is to form sets of blades or air foils which have a thrust reversing position.
U.S. Pat. No. 4,483,658 (Levine) shows a system where there are co-rotating sets of blades, positioned with one set in front of the other, these being arranged so that wake interceptions for a variety of relative velocities and blade spacing are arranged to provide acoustic and/or efficiency benefits.
U.S. Pat. No. 4,486,146 (Campion) shows a propeller arrangement where there is a propeller fan 27 which operates at a high-disc loading so that a substantial component of swirl is imparted to the slip stream. There is a multi-bladed stator which imparts a component of swirl in an opposite direction so that the total swirl imparted to the slip stream is nil or relatively low.
U.S. Pat. No. 4,621,978 (Stuart) shows a counter-rotating power turbine. This patent is directed particularly to the manner in which the rotating components are mounted.